Back hoe or grader

ABSTRACT

A back hoe or grader has a base member pivotally supporting a boom, and a dipper pivotally mounted at the end of the boom, the effective lengths of the boom and dipper being equal. Separate hydraulic devices swing the boom about its pivotal support and swing the dipper relative to the boom, and the controls for these hydraulic devices may be selectively connected so that the angular rate of travel of the dipper with respect to the boom is substantially twice the angular rate of travel of the boom about its pivotal support. This limits the motion of the work implement connection at the outer end of the dipper so that it cannot move below a preselected straight line. Links pivotally connected to the work implement are also connected to a movable pivot on the boom, so that the attitude of the work implement may remain constant with respect to the dipper for back hoe operations, or constant with respect to the grade being cut for grading operations. A subframe is mounted for pivotal movement on the base member, and this subframe supports the hydraulic device for swinging the boom about its pivotal support. An additional hydraulic device swings the subframe relative to the base member.

United States Patent [72] lnventor DonSuverkrop Bakersfield,Cali1. [21] Appl.No. 808,441 [22] Filed Mar. 19,1969 [45] Patented Apr. 27, 1971 [73} Assignee Hopper, Inc.

Bakersfield,Calif.

[54] BACK HOEOR GRADER 12 Claims, 13 Drawing Figs.

[52] U.S.Cl 214/138 [51] lnt.Cl E02i3/75 [50] FieldofSearch 214/138, 131,132,133;37/103 [5 6] References Cited UNITED STATES PATENTS 2,897,986 8/1959 Davis 214/131 3,165,842 1/1965 Holopainen. 214/138X 3,343,693 9/1967 Becker 214/138 3,395,816 8/1968 Hodgesetal.... 214/138 3,412,880 11/1968 Tweedale 214/138 3,463,335 8/1969 Brownelletal 214/138 FOREIGN PATENTS 987,637 2/1964 GreatBritain.....'. 214/730 Primary Examiner-Gerald M. Forlenza Assistant ExaminerLawrence J. Oresky Attorney-Lyon & Lyon ABSTRACT: A back hoe or grader has a base member pivotally supporting a boom, and a dipper pivotally mounted at the end of the boom, the effective lengths of the boom and dipper being equal. Separate hydraulic devices swing the boom about its pivotal support and swing the dipper relative to the boom, and the controls for these hydraulic devices may be selectively connected so that the angular rate of travel of the dipper with respect to the boom is substantially twice the angular rate of travel of the boom about its pivotal support. This limits the motion of the work implement connection at the outer end of the dipper so that it cannot --move below a preselected straight line. Links pivotally connected to the work implement are also connected to a movable pivot on the boom, so that the attitude of the work implement may remain constant with respect to the dipper for back hoe operations, or constant with respect to the grade being cut for grading operations. A subframe is mounted for pivotal movement on the base member, and this subframe supports the hydraulic device for swinging the boom about its pivotal support. An additional hydraulic device swings the subframe relative to the base member.

PATENTED mam 3576268 SHEET 1 BF 5 g It INVENTOR Pd/V SUI 6,6660

A TTOfA/(EVS PATENTEU APR27 1971 SHEET 2 [IF 5 5 v, w mm 5 0 W v V 0 M 5AM PATENTED mm 1971 SHEET 3 BF 5 i Wm I L ii In M INVENTOR DOA/5014556601 BACK nor: R GRADER This invention relates to earthmoving apparatus and is particularly directed to improvements in devices known as back hoes or graders. Such devices commonly employ an earth-engaging implement such as a bucket mounted at the swinging end of a dipper, which in turn is pivotally mounted on the swinging end of a boom. The boom is pivotally mounted on a base member. In conventional devices of this type, separate controls are provided for moving the dipper and for raising and lowering the boom. Therefore, the digging of a trench with a straight horizontal bottom surface or the grading of a surface to form an inclined plane, for example, requires a high degree of skill of the operator in manipulating both of the controls simultaneously. It is an object of this invention to provide a back hoe or grader of the boom and dipper type in which an adjustable control mechanism limits the extent of motion of the boom to prevent the ground-engaging implement from moving beyond the desired contour.

This general object is achieved by making the effective length of the dipper equal to the effective length of the boom, and interconnecting the separate hydraulic devices for moving each of them in a manner so that the angular rate of travel of the dipper with respect to the boom is substantially twice the angular rate of travel of the boom about its pivotal support, thereby insuring that the work implement connection at the outer end of the dipper is caused to move in a straight line.

Another feature of the present invention resides in the provision of linkage pivotally connected to the boom for controlling the attitude of the work implement at the end of the dipper.

Another feature of this invention increases the useful arc of travel of the boom beyond the usual maximum limit which can be achieved by a single hydraulic cylinder assembly, thereby increasing the range of operation of the entire device. This feature is achieved through the use of a subframe mounted for pivotal movement coaxial with the pivotal support for the boom.,The hydraulic cylinder assembly for swinging the boom is connected to the subframe. An additional hydraulic device swings the subframe relative to the base member which supports the boom. Pivotal movement of the subframe is thus employed for increasing the range of action of the boom actuating hydraulic cylinder assembly.

Other objects and advantages will appear hereinafter.

In the drawings:

FIG. 1 is a side elevation showing a preferred embodiment of this invention.

FIG. 2 is a top plan view thereof.

FIG. 3 shows a portion of the apparatus of FIG. 2, shown on an enlarged scale.

FIG. 4 is an enlargement of a portion of FIG. 1.

FIG. 5 is an enlargement of another portion of FIG. 2.

FIG. 6 is a diagram showing the principle involved in producing the straight line motion, the parts being shown in an extended position.

FIG. 7 is a diagram similar to FIG. 6, the parts being shown in a more retracted position.

FIG. 8 is a partial view, showing a modification.

FIG. 9 is a sectional view taken on line 99 as shown on FIG. 8.

FIGS. 10, 11, 12 and 13 are hydraulic circuit diagrams.

Referring to the drawings, the back hoe or grader device generally designated 10 includes a base member 11 which may be mounted on a truck 12 for turning movement about the axis of the supporting bearing 13. The boom 15 is pivotally supported at 16 on the base member 11. The pivotal support 16 is referred to hereinafter as the first pivot." The boom 15 carries a second pivot 17 which connects the boom to the dipper 18. A third pivot 19 at the swinging end of the dipper 18 connects the dipper 18 to the lever 20 of the work implement generally designated 21.

The first power cylinder assembly 23 is pivotally connected to the boom 15 at 24 and serves to swing the boom 15 around the pivot 16. A second power cylinder assembly 25 is pivotally connected to the boom 15 at 26 and is pivotally connected to the dipper 16 at 27, and serves to swing the dipper 16 with respect to the boom 15 around the pivot 17.

A subframe 30 is pivotally mounted on the base member 11 for pivotal movement around the axis of the pivot 16. Duplicate hydraulic cylinder assemblies 31 are mounted on opposite sides of the subframe 30 and each has one end pivotally connected to the base member 11 at 32 and has the other end pivotally connected at 33 to a bracket 34 fixed to the subframe. Accordingly, the angular position of the subframe 30 with respect to the base member 11 is controlled by the hydraulic cylinder assemblies 31.

The work implement 21 is of conventional construction and includes a bucket 36 having a digging edge 37 and mounted for swinging movement about the longitudinal axis of the block 38. This swinging movement is controlled by a rotary hydraulic device 39. The block 38 is pivotally supported on the lower ends of the levers 20 and turning motion of the block 38 about the transverse axis 40 is controlled by means of the rotary hydraulic devices 41. Conventional hydraulic hoses communicating with the devices 39 and 41 have been omitted from the drawings for clarity of illustration.

The parallel levers 20 are pivotally connected at 43 to the links 44 which serve to control the attitude of the work implement 21 during operation of the device. The rearward end of each link 44 is connected by pin 45 to a slide block 46 mounted to travel between parallel guides 47 fixed to the boom 15. The proportions of the parts are such that the blocks 46 may be moved from the position shown in FIG. 4 to another position in which the pins 45 are coaxial with the pivot 17 which connects the boom 15 and dipper 18. Movement of each block 46 is controlled by power cylinder assembly 49 pivoted at one end at 50 to the boom 15 and pivoted at the other end at 51 to the block 46. For grading operations, the pins 45 are held in the position shown in FIG. 4, so that the links 44 tend to hold the work implement 21 in approximately the same attitude with respect to the surface being cut. For back hoe operations, it is desirable that the implement 21 remain in fixed attitude with respect to the dipper l8, and for this purpose the pins 45 are moved into coaxial position with the pivot 17.

The diagrams of FIGS. 6 and 7 illustrate the geometric principle which produces the straight line motion of the pivot 19 at the swinging end of the dipper lb. The effective length B of the boom 15 is equal to the effective length D of the dipper 18. These effective lengths constitute the straight line distance between the pivots at the opposite ends of the boom and the dipper. In FIG. 6, the angle c is equal to twice the angle a. The angle 6 is measured between a line connecting the pivots l7 and 19, and another line connecting the pivots 16 and 17. The angle a is measured between a first line passing through pivot 16 and perpendicular to the line passing through pivots 116 and 19, and a second line connecting the pivots 16 and 17. Similarly, in Flg. 7, the angle c is equal to twice the angle a,. In order for the pivot 19 to travel in a straight line from the position shown in FIG. 6 to the position shown in FIG. 7, it is necessary that the angle 0 at all positions of boom 15 and dipper 18 be equal to twice the angle a. Expressed in other tenns, the angular rate of travel of the dipper with respect to the boom is twice the angular rate of travel of the boom about its pivotal support.

In order to coordinate the operation of the power cylinder assembly 25 with the operation of the power cylinder assemblies 23 and 31, there is provided a sprocket or circular member 53 positioned coaxially of the pivot 17 and connected to the dipper 18 for turning movement therewith. A larger sprocket 54 or circular member is mounted coaxially of the pivot 16. This sprocket 54 has an effective diameter twice that of the sprocket 53. The sprockets are connected by the endless flexible chain 55. The sprocket 54 turns freely on the shaft 58 which extends coaxially of the pivot 16. A hub 59 is fixed to the shaft 58 and an axially movable pin 60 is carried on the hub and arranged to engage any one of a series of openings provided in the sprocket 54. The pin 60 serves as a means to fix the sprocket 54 to the shaft 58 at any one of a plurality of angular positions.

A portion of the shaft 58 is externally splined for sliding reception of an internally splined sleeve 61. A shifter rod 62 extends from the operator's position on the base member 11 to the shifter fork 63 engaging the sleeve 61. The arm 64 fixed to the sleeve 61 carries a servo pilot valve at its outer end operated by cam follower roller 65. This roller 65 engages the outer periphery of any one of a series of earns 66, 67 fixed on the cam shaft 68. The hydraulic motor 70 of the positive displacement type drives the shaft 68 through sprockets 71 and 72 and chain 73. This hydraulic motor 70 is connected in series with one of the hydraulic lines 75 leading to hydraulic cylinder assembly 25. The servo pilot valve is connected to modify the control of the boom cylinder assemblies 23 and 31, as shown in FIG. 12. The full stroke of the hydraulic cylinder assembly 25 causes 270 of rotation of the cam shaft 68 and cams 66, 67. A cam 67 comprises a circular disc concentric with the axis of the shaft 68. Such a cam is employed when the movement desired for the pivot 19 is a straight line motion toward and away from the pivot 16. The cam 66 has a generated shape which will cause the pivot 19 to move in a straight line not intersecting the pivot 16; for example, when digging a deep trench with a horizontal bottom surface. The sleeve 61 may be shifted axially along the spline shaft 58 by means of the control rod 62 in order to bring the cam follower roller 65 under the selected cam.

From the foregoing description it will be understood that the position of the pin 60 with respect to the sprocket 54 controls the angle of the cut, whereas the shape of the cam 66 controls the maximum depth of the cut. The operator always has completely independent control of the dipper 18. For each position of the dipper there is a downward limit on the position of the boom which limit is established when the cam roller 65 comes in contact with the cam 66. But even while the cam roller 65 is in contact with the cam 66, the operator maintains complete control save for an opposing pressure he will feel on his control lever for boom 15 whenever such limiting depth is reached.

For certain types and depth of operation, it may be desirable to have a boom and dipper of other than equal length. In this situation a modified cam 66 may be installed to compensate for the inequality in length, and provided with a profile to give a straight bucket traverse, or other desired contour. If desired, a barrel" cam 95 of the type shown in FIGS. 8 and 9 may be substituted for the earns 66 and 67. One end 96 of this barrel cam may be circular and the other end 97 profiled to produce the maximum depth of cut. At intermediate positions between the ends 96 and 97, the cam roller 65 limits the depth of cut to intermediate depths.

The various hydraulic circuits of FIGS. 10-13 are shown diagrammatically. FIG. 10 shows control valves 80 and 81 for operating the bucket-manipulating devices 39 and 40, respectively. FIG. 11 shows valve 82 for operating the cam-tuming motor 70 and the hydraulic cylinder assembly 25 which swings the dipper 18. FIG. 12 shows valve 84 which actuates the boom-operating cylinder assemblies connected in parallel and shown diagrammatically at 23, 31. The servo pilot valve 85 is carried at the end of the arm 64 and is operated by the cam follower roller 65. ln FlG. 13, the valve 87 operates the hydraulic motor 88 for turning the base member 11 by means of the pinion gear 89 and ring gear 90. The controls for operating the valves 80, 81, 82, 84 and 87, as well as the valve for operation of the hydraulic cylinder assemblies 49 are mounted in a panel 92 in a convenient location for the operator.

When it is desired to use the device as a back hoe, the hydraulic cylinder assemblies 49 are actuated to move the pins 45 into coaxial alignment with the pivot 17, and the hydraulic devices 39 and 41 are actuated to bring the bucket 21 into the desired angular position. The shifter lever 62 is moved to bring the cam follower roller 65 under the proper cam for the depth of cut to be made. Pin 60 is set to establish the desired angular relation of the bottom out to the base member 11. The two levers for actuating the valves 82 and 84 are then independently manipulated to move the dipper 18 and boom 15 as desired. When the desired depth of cut has been achieved, corresponding to the particular cam 66 selected, cam roller 65 will come into contact with cam 66 placing a downstroke limit on boom 15 for each position of dipper 18. The operator retains independent control of the boom and dipper but will reach a limiting depth line or contour below which he can not dig.

It will be observed that, although the sprockets 53 and 54 and connecting chain 55, as well as sprockets 71, 72 and connecting chain 73, are employed in coordinating the motions of the boom 15 and dipper 18, they are not subjected to any substantial forces, and therefore may be relatively small size.

It will also be observed that the pivoted subframe 30 and cylinder assemblies 31 increase the range of angular movement of the boom 15, so that it may swing through a fulloperating range and also reach a retracted folded position over the base member 11 for transport, as shown by the phantom lines in FIG. 1.

When the device is to be used as a grader, the hydraulic cylinder assemblies 49 are actuated to move the pins 45 to the position shown in FIGS. 1, 2 and 4. The links 44 are then effective to cause pivotal movement of the work implement lever 20 to hold the cutting edge 37 of the bucket 36 in substantially constant angular relationship with the grade being cut. Further steps in the operation of the device are similar to those described above.

Having fully described my invention, it is to be understood that I am not to be limited to the details herein set forth but that my invention is of the full scope of the appended claims.

lclaim:

1. In a back hoe or grader, the combination of: a boom having a pivotal support at one end, a dipper having one end pivotally connected to the other end of the boom, a work implement including a lever, pivotal means at the other end of the dipper for connection to said lever, the boom and the dipper being of substantially the same length, power means for swinging the boom about its pivotal support, second power means for swinging the dipper with respect to the boom, means interconnecting said power means to coordinate their simultaneous action so that the angular rate of movement of the dipper with respect to the boom is substantially twice the angular rate of movement of the boom about its pivotal support, whereby the connection means for the work implement is caused to move in a straight line, pivot means on said boom, and a link connected at one end to said pivot means and pivotally connected at the other end to said lever.

2. The combination set forth in claim 1 in which power means are provided for moving said pivot means relative to said boom.

3. ln a back hoe or grader, the combination of: a base member, a boom mounted on the base member for movement in a vertical plane about a first pivot, a dipper mounted on the boom for movement in the same vertical plane about a second pivot, a work implement mounted at the swinging end of said dipper for movement about a third pivot, the distance between the first and second pivots being substantially equal to the distance between the second and third pivots, means including a first hydraulic cylinder assembly for swinging said boom with respect to said base member, means including a second hydraulic cylinder assembly for swinging said dipper with respect to said boom, means interconnecting said hydraulic cylinder assemblies to coordinate their simultaneous movements so that the dipper travels at substantially twice the angular rate about said second pivot as the boom travels about said first pivot, whereby the third pivot is caused to move in a straight line, said work implement including a lever connected to the dipper by means of said third pivot, pivot means on said boom, and a link connected at one end to said pivot means and pivotally connected at the other end to said lever.

4. The combination set forth in claim 3 in which power means are provided for moving said pivot means relative to said boom.

5. In a back hoe or grader, the combination of: a base member, a boom mounted on the base member for movement in a vertical plane about a first pivot, a dipper mounted on the boom for movement in the same vertical plane about a second pivot, a work implement mounted at the swinging end of said dipper for movement about a third pivot, the distance between the first and second pivots being substantially equal to the distance between the second and third pivots, means for swinging said boom with respect to said base member, said means including a subframe pivoted to said base member, means for swinging the subframe relative to the base member, a first hydraulic cylinder assembly operatively interposed between said subframe and said boom, means including a second hydraulic cylinder assembly for swinging said dipper with respect to said boom, and means interconnecting said hydraulic cylinder assemblies to coordinate their simultaneous movement so that the dipper travels at substantially twice the angular rate about said second pivot as the boom travels about said first pivot, whereby the third pivot is caused to move in a straight line.

6. In a back hoe or grader, the combination of: a base member, a boom having a pivotal support at one end on the base member, a dipper having one end pivotally connected to the other end of the boom, means at the other end of the dipper for connection to a work implement, the boom and the dipper being of substantially the same length, power means on the base member for swinging the boom about its pivotal support, second power means for swinging the dipper with respect to the boom, and means interconnecting said power means to coordinate their simultaneous action so that the angular rate of movement of the dipper with respect to the boom is substantially twice the angular rate of movement of the boom about its pivotal support, said interconnecting means including a first sprocket coaxial with said pivotal support, a second sprocket coaxial with the pivotal connection between the boom and the dipper, and connected to turn with the dipper, the first sprocket having twice the effective diameter of the second sprocket, an endless chain connecting said sprockets, and means responsive to relative motion between the first sprocket and the base member for modifying the action of the first said power means.

7. in a back hoe or grader, the combination of: a base member, a boom mounted on the base member for movement in a vertical plane about a first pivot, a dipper mounted on the boom for movement in the same vertical plane about a second pivot, a work implement mounted at the swinging end of said dipper for movement about a third pivot, the distance between the first and second pivots being substantially equal to the distance between the second and third pivots, means including afiist hydraulic cylinder assembly for swinging said boom with respect to said base member, means including a second hydraulic cylinder assembly for swinging said dipper with respect to said boom, and means interconnecting said hydraulic cylinder assemblies to coordinate their simultaneous movements so that the dipper travels at substantially twice the angular rate about said second pivot as the boom travels about of the second sprocket, and endless chain connecting said sprockets, and means responsive to relative motion between the first sprocket and the base member for modifying the action of said first hydraulic cylinder assembly.

8. in a back hoe or grader, the combination of: a base member, a boom pivotally supported on the base member, a dipper pivotally connected to the boom, hydraulic power means on the base member for swinging the boom about its pivotal support, second hydraulic power means for swinging the dip with respect to the boom, means for limiting the extent o pivotal motion of the boom in one direction, said means including a cam mounted to turn on the base member, means for turning the cam in accordance with movement of the second hydraulic power means, a follower adapted to contact the cam, support means for moving the follower in accordance with relative pivotal movement of the boom and the dipper, means including valve means for operating the first hydraulic means, said valve means including a servo pilot valve operatively interposed between said follower and its support means, whereby contact of the follower and cam opens the servo pilot valve.

9. in a back hoe or grader, the combination of: a base member, a boom pivotally supported on the base member, a dipper pivotally connected to the boom and having a work implement on the swinging end thereof, hydraulic power means on the base member for swinging the boom about its pivotal support, second hydraulic power means for swinging the dipper with respect to the boom, a cam mounted to turn on the base member, means for turning the cam in accordance with movement of the second hydraulic power means, a follower adapted to contact the cam, support means for moving the follower in accordance with relative pivotal movement of the boom and the dipper, means including valve means for operating the first hydraulic means, said valve means including a servo pilot valve operatively interposed between said follower and its support means, whereby downward pivotal movement of the boom is limited by downward movement of the implement to a predetermined position.

10. The combination set forth in claim 9 in which the follower support means includes an arm mounted to turn about the axis of the boom pivot.

11. The combination set forth in claim 9 in which the follower support means includes an arm mounted to turn about the axis of the boom pivot, a first circular member coaxial with the boom pivot and connected to drive the arm, a second circular member coaxial with the pivotal connection between the boom and the dipper and connected to turn with the dipper, and a flexible element connecting the circular members.

12. in combination, a base member, a first boom mounted on the base member for movement in a vertical plane about a first pivot, a second boom mounted on the first boom for movement in the same vertical plane about a second pivot, a work member mounted at the swinging end of the second boom for movement about a third pivot, the distance between the first and second pivots being substantially equal to the distance between the second and third pivots, a first circular member mounted coaxially of the first pivot, a second circular member mounted coaxially of said second pivot and connected to turn with said second boom, said first circular member having twice the effective diameter of said second circular member, a flexible element connecting said circular members, and means for limiting turning movement of said first circular member. 

1. In a back hoe or grader, the combination of: a boom having a pivotal support at one end, a dipper having one end pivotally connected to the other end of the boom, a work implement including a lever, pivotal means at the other end of the dipper for connection to said lever, the boom and the dipper being of substantially the same length, power means for swinging the boom about its pivotal support, second power means for swinging the dipper with respect to the boom, means interconnecting said power means to coordinate their simultaneous action so that the angular rate of movement of the dipper with respect to the boom is substantially twice the angular rate of movement of the boom about its pivotal support, whereby the connection means for the work implement is caused to move in a straight line, pivot means on said boom, and a link connected at one end to said pivot means and pivotally connected at the other end to said lever.
 2. The combination set forth in claim 1 in which power means are provided for moving said pivot means relative to said boom.
 3. In a back hoe or grader, the combination of: a base member, a boom mounted on the base member for movement in a vertical plane about a first pivot, a dipper mounted on the boom for movement in the same vertical plane about a second pivot, a work implement mounted at the swinging end of said dipper for movement about a third pivot, the distance between the first and second pivots being substantially equal to the distance between the second and third pivots, means including a first hydraulic cylinder assembly for swinging said boom with respect to said base member, means including a second hydraulic cylinder assembly for swinging said dipper with respect to said boom, means interconnecting said hydraulic cylinder assemblies to coordinate their simultaneous movements so that the dipper travels at substantially twice the angular rate about said second pivot as the boom travels about said first pivot, whereby the third pivot is caused to move in a straight line, said work implement including a lever connected to the dipper by means of said third pivot, pivot means on said boom, and a link connected at one end to said pivot means and pivotally connected at the other end to said lever.
 4. The combination set forth in claim 3 in which power means are provided for moving said pivot means relative to said boom.
 5. In a back hoe or grader, the combination of: a base member, a boom mounted on the base member for movement in a vertical plane about a first pivot, a dipper mounted on the boom for movement in the same vertical plane about a second pivot, a work implement mounted at the swinging end of said dipper for movement about a third pivot, the distance between the first and second pivots being substantially equal to the distance between the second and third pivots, means for swinging said boom with respect to said base member, said means including a subframe pivoted to said base member, means for swinging the subframe relative to the base member, a first hydraulic cylinder assembly operatively interposed between said subframe and said boom, means including a second hydraulic cylinder assembly for swinging said dipper with respect to said boom, and means interconnecting said hydraulic cylinder assemblies to coordinate their simultaneous movement so that the dipper travels at substantially twice the angular rate about said second pivot as the boom travels about said first pivot, whereby the third pivot is caused to move in a straight line.
 6. In a back hoe or grader, the combination of: a base member, a boom having a pivotal support at one end on the base member, a diPper having one end pivotally connected to the other end of the boom, means at the other end of the dipper for connection to a work implement, the boom and the dipper being of substantially the same length, power means on the base member for swinging the boom about its pivotal support, second power means for swinging the dipper with respect to the boom, and means interconnecting said power means to coordinate their simultaneous action so that the angular rate of movement of the dipper with respect to the boom is substantially twice the angular rate of movement of the boom about its pivotal support, said interconnecting means including a first sprocket coaxial with said pivotal support, a second sprocket coaxial with the pivotal connection between the boom and the dipper, and connected to turn with the dipper, the first sprocket having twice the effective diameter of the second sprocket, an endless chain connecting said sprockets, and means responsive to relative motion between the first sprocket and the base member for modifying the action of the first said power means.
 7. In a back hoe or grader, the combination of: a base member, a boom mounted on the base member for movement in a vertical plane about a first pivot, a dipper mounted on the boom for movement in the same vertical plane about a second pivot, a work implement mounted at the swinging end of said dipper for movement about a third pivot, the distance between the first and second pivots being substantially equal to the distance between the second and third pivots, means including a first hydraulic cylinder assembly for swinging said boom with respect to said base member, means including a second hydraulic cylinder assembly for swinging said dipper with respect to said boom, and means interconnecting said hydraulic cylinder assemblies to coordinate their simultaneous movements so that the dipper travels at substantially twice the angular rate about said second pivot as the boom travels about said first pivot, said interconnecting means including a first sprocket coaxial with said first pivot, a second sprocket coaxial with said second pivot and connected to turn with said dipper, the first sprocket having twice the effective diameter of the second sprocket, and endless chain connecting said sprockets, and means responsive to relative motion between the first sprocket and the base member for modifying the action of said first hydraulic cylinder assembly.
 8. In a back hoe or grader, the combination of: a base member, a boom pivotally supported on the base member, a dipper pivotally connected to the boom, hydraulic power means on the base member for swinging the boom about its pivotal support, second hydraulic power means for swinging the dipper with respect to the boom, means for limiting the extent of pivotal motion of the boom in one direction, said means including a cam mounted to turn on the base member, means for turning the cam in accordance with movement of the second hydraulic power means, a follower adapted to contact the cam, support means for moving the follower in accordance with relative pivotal movement of the boom and the dipper, means including valve means for operating the first hydraulic means, said valve means including a servo pilot valve operatively interposed between said follower and its support means, whereby contact of the follower and cam opens the servo pilot valve.
 9. In a back hoe or grader, the combination of: a base member, a boom pivotally supported on the base member, a dipper pivotally connected to the boom and having a work implement on the swinging end thereof, hydraulic power means on the base member for swinging the boom about its pivotal support, second hydraulic power means for swinging the dipper with respect to the boom, a cam mounted to turn on the base member, means for turning the cam in accordance with movement of the second hydraulic power means, a follower adapted to contact the cam, support means for moving the follower in accordance with relative pivotal moveMent of the boom and the dipper, means including valve means for operating the first hydraulic means, said valve means including a servo pilot valve operatively interposed between said follower and its support means, whereby downward pivotal movement of the boom is limited by downward movement of the implement to a predetermined position.
 10. The combination set forth in claim 9 in which the follower support means includes an arm mounted to turn about the axis of the boom pivot.
 11. The combination set forth in claim 9 in which the follower support means includes an arm mounted to turn about the axis of the boom pivot, a first circular member coaxial with the boom pivot and connected to drive the arm, a second circular member coaxial with the pivotal connection between the boom and the dipper and connected to turn with the dipper, and a flexible element connecting the circular members.
 12. In combination, a base member, a first boom mounted on the base member for movement in a vertical plane about a first pivot, a second boom mounted on the first boom for movement in the same vertical plane about a second pivot, a work member mounted at the swinging end of the second boom for movement about a third pivot, the distance between the first and second pivots being substantially equal to the distance between the second and third pivots, a first circular member mounted coaxially of the first pivot, a second circular member mounted coaxially of said second pivot and connected to turn with said second boom, said first circular member having twice the effective diameter of said second circular member, a flexible element connecting said circular members, and means for limiting turning movement of said first circular member. 